Personally, I subscribe to the utilitarian theory of ethics. Since I am not a philosopher, I'll quote wikipedia:
Utilitarianism is both a metaethical doctrine, and a theory in normative ethics. Utilitarianism holds, in its simplest form, that "the good" is whatever yields the greatest "utility". Utility has been understood in different ways - happiness, pleasure, preference-satisfaction, etc. - but it is always a naturalistic conception of an individual's good. As a metaethical doctrine, it holds that "whatever yields the greatest utility" is the meaning of the word "good" (thus it is a naturalistic theory of metaethics); while as a normative theory, it merely holds that "whatever yields the greatest utility" is in fact good, whatever the meaning of the word "good" may be.
Basically, a good act is one which causes the greastest amount of pleasure to most people*, while an evil one causes the greatest amount of pain.
*Some, myself included, would include animals.
[edit] linky
http://utilitarian.org/one.html